4.9.9 Points to Remember About getline

Here are some miscellaneous points about getline that
you should bear in mind:

When getline changes the value of $0 and NF,
awk does not automatically jump to the start of the
program and start testing the new record against every pattern.
However, the new record is tested against any subsequent rules.

Many awk implementations limit the number of pipelines that an awk
program may have open to just one. In gawk, there is no such limit.
You can open as many pipelines (and coprocesses) as the underlying operating
system permits.

An interesting side effect occurs if you use getline without a
redirection inside a BEGIN rule. Because an unredirected getline
reads from the command-line data files, the first getline command
causes awk to set the value of FILENAME. Normally,
FILENAME does not have a value inside BEGIN rules, because you
have not yet started to process the command-line data files.
(d.c.)
(See BEGIN/END,
also see Auto-set.)

Using FILENAME with getline
(‘getline < FILENAME’)
is likely to be a source for
confusion. awk opens a separate input stream from the
current input file. However, by not using a variable, $0
and NR are still updated. If you’re doing this, it’s
probably by accident, and you should reconsider what it is you’re
trying to accomplish.

Getline Summary, presents a table summarizing the
getline variants and which variables they can affect.
It is worth noting that those variants which do not use redirection
can cause FILENAME to be updated if they cause
awk to start reading a new input file.

If the variable being assigned is an expression with side effects,
different versions of awk behave differently upon encountering
end-of-file. Some versions don’t evaluate the expression; many versions
(including gawk) do. Here is an example, due to Duncan Moore:

Here, the side effect is the ‘++c’. Is c incremented if
end of file is encountered, before the element in a is assigned?

gawk treats getline like a function call, and evaluates
the expression ‘a[++c]’ before attempting to read from f.
Other versions of awk only evaluate the expression once they
know that there is a string value to be assigned. Caveat Emptor.